MUSEUM OF COMrAEATIVE ZOOLOGY. 119 



bombs. Above the tufaceous deposit is a sheet of very amygdaloidal 

 trap, overlain by a dark pinkish gray sandstone, carrying two thin sub- 

 ordinate layers of trappy material a few feet over the contact. In the 

 hand specimen and under the microscope, this sandstone appears identi- 

 cal with the fine matrix of a trap conglomerate noted by Percival as 

 occurring half a mile to the north, and presumably forming the strati- 

 gi'aphical equivalent of the tufaceous deposit at this point. The se- 

 quence of outcrops here disclosed is one of the most valuable that it has 

 been our fortune to discover, and has attracted much local attention 

 since it was found in the spring of 1887. It will well repay attentive 

 examination. The following account refers in greater part to its micro- 

 scopic structures. 



Under the microscope the material of the bluflf enclosing the volcanic 

 bombs is found to be made up of small fragments of trap, generally very 

 fine-grained and much altered. Small greenish brown areas dotted 

 thickly with ferrite are non-polarizing as a whole ; these appear to be 

 volcanic glass. A few porphyritic ledges of plagioclase occur in them. 

 Most of the eruptive grains have been altered to chlorite and quartz, 

 and are intimately mixed with granular calcite. The microscope fails 

 to discover any grains of water-worn quartz or other clastic material, 

 although it is probable that moi-e or less normally deposited sediment 

 occurs thinly scattered through the mass. No stratified arrangement 

 of the trap grains is noticeable in the microscopic sections, except an 

 orientation of chloi'ite plates parallel to the stratification of the sand- 

 stone on the back of the ridge, and to a rude lamination brought to sight 

 in the face of the tufaceous bed by weathering. Following Geikie, this 

 bed would be called a tufa, consisting of a shower of lapilli. It appears 

 to have been deposited rather rapidly in a body of water, and probably 

 at no great distance from a point of eruption, as it soon disappears to 

 the north and west. It is traceable a mile and a half to the southeast, 

 in localities 6 and 7. 



The volcanic bombs occurring with tlie lapilli give the face of the 

 bluflf a curious mottled appearance. They show no definite arrangement, 

 but are more ninnerous near the bottom of the bed, where one of them 

 seems to have imbedded itself in the underlying sandstone ; they are 

 remarkable for their non-vesicular character and their compact uniform 

 texture from the centre to the surface. The microscope detects no 

 variation in texture in any part except that due to a partial alteration 

 of the surface. It shows them to be extremely close-grained, with por- 

 phyritic crystals of augite set in a ground mass of minute plagioclase 



