202 



ALBERT D. BROKAW AND LEON P. SMITH 



The partly altered material is notably lighter in color than the 



fresh rock. Most of the 

 hornblende has been 

 weathered, leaving brown 

 iron stains and a small 

 amount of chlorite, but 

 some particles of appar- 

 ently fresh hornblende 

 are to be seen. The feld- 

 spar has been changed to 

 a chalky white material 

 in an iron-stained matrix. 

 Microscopical study re- 

 veals some unchanged 

 hornblende, but for the 

 most part only alteration 



, , , ., ,, , ,, products are discernible. 

 Fig. 2. — Section of fresh hornblende gabbro * 



X20. (Two rather large cracks appear as light Limonite, white mica 

 bands.) Ordinary light. (probably also gibbsite), 



chlorite, zoisite, and a 

 small quantity of mag- 

 netite make up the mass. 

 The weathered zone is 

 sharply set off from the 

 fresh material even in thin 

 section. In Fig. 3 the 

 fresh portion is shown on 

 the left, changing to 

 altered on the right. 



The most completely 

 weathered portion is a fri- 

 able, earthy, non-plastic 

 mass, strongly iron 

 stained, and so thoroughly 

 disintegrated that the 

 original texture of the 



Fig. 3. — Slide showing transition from fresh to 

 altered portion of specimen. X25. Crossed nicols. 



