R<K-l-x near Hcatheote. 325 



A rock section (No. 602) shoAvs that no original minerals 

 remain, the rock now consisting of chlorite, carbonates, chalce- 

 dony, secondary quartz and secondary albite. Some of the 

 original structures remain, however, and the presence of amygda- 

 loidal cavities filled with secondaiy minerals is noticeable, not 

 only in the rnck section, but abundantly in the field, and demon- 

 strates that the rock was originally a vesicular lava flow. 



Pyroclastic Bocks. — Fragmental igneous rocks in my opinion 

 represent the predominant type in the area I have examined. 

 They include agglomerates, tuffs and other altered rocks, origin- 

 ally composed mainly, if not entirely, of diabasic fragments 

 which may be tuft's or detrital sediments. The latter are gene- 

 rally well bedded, and usually more or less completely altered to 

 cherts. 



Agglomerates. — The best example of these occurs at Photo- 

 graph Knob, in S. Heathcote. A small eminence here marks the 

 position of the plug of an old volcanic rent, in and near which 

 the coarser diabase fragments fell. Subsequent alteration has 

 converted the mass into a very tough, hard rock. The agglome- 

 rate, from its relation to finer tuffs exposed in a stream bed 

 to the S., and also seen to the north, appears to be rather 

 younger than and intrusive into them. My interpretation of 

 relations of the rocks near here is shown in the accompanying 

 sketch section (PI. XYTT.. Fig. 1). Under the microscope (No. 545) 

 (PI. XIV., Fig. 2) the agglomerate consists of angular fragments of 

 a dark basaltic rock with lath-shaped felspars in a groundmass of a 

 more felspathio reciystallised diabase, possibly originally frag- 

 mental. Lt consists now entirely of secondary' minerals, includ- 

 ing felspars, quartz and needle-shaped aotinolite. Vivid green 

 chlorite fills cavities in the rocks, and also replaces primary 

 minerals, probably felspar and augite. The large size of some 

 of the rock fragments gives the rock the character of a typical 

 agglomerate. 



Another rock (No. 576) (PL XIV., Fig. 3) occurring as boulders 

 in the foliated diabase of Red Hill, near the junction with the 

 granitic rock, is also coarsely fragmental in character. Under 

 the microscope some of the fragments are seen to be similar to 

 the platy diabase. They are set. in a dense groundmass whose 

 character is obscured by alteration, and the boundaries of the 

 frasrments are defined bv a red-brown ferrucrinous laver. 



