QUOTES

"All around him the long scar smashed into the jungle was a bath of heat."

• Alliteration twice - emphasis of phrase therefore an indication that it is significant to the reader.
• The sibilance "scar smashed' introduces the symbol using negative language as well as evoking connotations such as 'scorch' which links to the theme of heat and evil.

The scar (literally) is a demolished line of trees from where the plane crashed. However figuratively that the noun 'scar' used in this quote is personification of the island (the human attribute of being hurt - scarred). Humans have the capacity to wound and scar nature much like they do with each other. The scar can therefore be seen as the evil mankind has wrought on the world. 

(Alternative interpretation) - We can also interpret the symbol of the scar as an allusion to original sin and mankind's fall in the Bible. The scar is an ugly mar on the island and as a symbol of original sin, it anticipates the boys fall from grace. The paradise that the boys discovered is imperect upon their arrival. Ironically it is the boys themselves that will ruin the paradisiacal landscape. Golding is foreshadowing the destruction the boys will bring.

'bath of heat' is a metaphor suggesting that the air in the clearing is so hot that its as though Ralph is in a bath.

"a bird, a vision of red and yellow, flashed upwards with a witch-like cry; and this was echoed by another."

• Visual and aural imagery 
• red and yellow are warning colours in nature, this insights danger and alerts the reader of bad omens to come.
• "witch-like cry' - ominous nature of the natural island is found in this similie. Alternatively it could be seen as foreshadowing (mention of witch implies evil). This predicts the boys savage '"ululation" (howling or wailing sound, in many cultures the sound is common at a funeral)  in Chapter 12 'cry of the hunters'. Golding is subtly hinting at the deaths to come.
• The noun 'vision' implies the intense heat is leading him to hallucinate, is Ralph really seeing the bird or is Golding trying to convey that the boys will be blind to their inherrent evil? 
• The verb 'flashed' implies the fast movement, is Golding suggesting how quickly the boys will descend to savagery? Or perhaps how quickly nature has dissmissed the boys presence, aware man will wreak havock?
• Nature is communictaing "echoed by another". The verb 'echoed' streses the enormity of nature and the island. The birds are alerting the island to the presence of man (evil).