32 HAEHL AND ARNOLD — THE MIOCENE DIABASE. LFeb. 5, 



puzzling. The layer is interbedded between shale layers and at 

 first was thought to be intrusive in the shale, but a later examina- 

 tion showed its true relation to the shales and its clastic origin. 



The typical tuff is found in thick beds all along the southwest- 

 ern and part of the northeastern side of the Langley Hill-Mindego 

 Hill ingneous area. Fig. 12a is a photograph of a hand-specimen 

 showing a weathered surface of the tuff, while 12I? shows a freshly 

 fractured surface. The fragments composing the tuff are of dark- 

 colored basaltic diabase, angular in outline and varying in size from 

 the smallest grains to large masses weighing several hundred pounds. 

 The slides of these fragments show them to be badly weathered, a 

 few feldspars, a little augite and the magnetite and ilmenite being 

 the only recognizable original constituents. The fragments are 

 imbedded in a limy matrix, varying in composition from pure linie 

 to a limy shale. Spheroidal weathering of the tuffs was noticed in 

 one or two instances. Small organic remains are often found asso- 

 ciated with the rock fragments in some of the more limy tuffs. 

 Much, and sometimes all, of the lime occurs in a secondary form, 

 as veins of calcite surrounding the fragments or cutting through 

 the tuff. Pure calcite crystals weighing several ounces are some- 

 times found in the tuff. This calcite is derived principally from 

 the original lime beds in which the tuff was deposited, but a little 



Fig. 13. Thin section of diabase tuff, showing secondary calcite vein, (C) ; 

 patches of secondary calcite, (C) ; feldspar, (F) ; magnetite, (M). X 20. 

 Photograph by Ralph Arnold. 



