ASSOCIATED ROCKS OF THE LIZARD DISTRICT. 



897 



feet thick, forms a sort of terrace, some height above the shore, 

 which can be reached with a little difficulty ; the gabbro is extremely 

 coarse, the diallage crystals being often two or three inches long ; 

 and some of the largest occur in a vein only a few inches thick. 

 Just on the left there appears to be a small fault in the serpentine ; 

 on the right is a large included mass of schist forming a headland, 

 down to which descends a dyke of dark compact trap about a foot 

 wide. This can be seen to cut through two sheets of gabbro and the 

 serpentine between them, and ends abruptly against the schist. The 

 examination of this part of the coast leads to the following con- 

 clusions : — 



1. That the serpentine is an intrusive rock. 



2. That probably the hornblende schist was metamorphosed prior 

 to its intrusion. 



3. That the gabbro was probably intruded when the serpentine 

 had arrived at its present condition. 



4. That the black trap dyke was intruded last of all. 



Under the microscope the trap is found to consist of a groundmass 

 generally microcrystalline, consisting probably of some pseudomorphic 



Fig. 4. — Gabbro intrusive in Serpentine and Hornblende Schist 



north of the Balk. 



A. Serpentine. 



B. Gabbro. 



C. Gabbro showing a foliated structure. 



D. Hornblende schist. 



product after plagioclase, with perhaps minute crystals of actinolite 

 enlarged, and a large number of small hornblende crystals, merely 

 rather platy in structure, but varying from the normal to the actino- 

 litic form. There are, as usual, grains of magnetite, and a number of 

 microliths, sometimes acicular, sometimes rather irregular in form, 

 which are commonly included in the larger hornblende crystals, lying 

 with their longer axes in the planes of principal cleavage. I have 

 not been able to satisfy myself as to the nature of these. At pre- 

 sent we must call the rock a diorite ; but whether it has always been 

 hornblendic is by no means certain. 



From the above headland a walk of about a quarter of a mile 

 along the edge of the cliffs leads to Polbarrow Cove. The cliffs, 



