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large plienocrysts of more or less altered plagioclase, often measuring half 
an inch across. These two groups of rock are connected by transitional 
varieties. They were xnobahly, in the first instance, composed of plagioclase, 
augite, iron-ores, and a variable quantity of imperfectly crystallized interstitial 
matter. 
Some of these rocks closely resemble in outward appearance the 
andesites (“ porphyrites ”) of the Old Eed Sandstone of the district not 
many miles to the north, that is, fine purplish-red rocks with a compact 
Fig. 51. — Structure in finely-aniygdaloidal diabase lava, south of mouth of Stiiichar River, Ayrsliire. 
The fine dots and circle.s mark the lines of amygdales. 
base through which porphyritic felspars are abundantly scattered. Occasion- 
ally they are markedly slaggy, and show even a ropy surface, while the 
breccias associated with them contain blocks of similar slag. 
But the most characteristic external feature of these lavas is their 
tendency to assume irregularly-elliptical, sack-like or pillow-shaped forms 
On a weathered face they sometimes look like a pile of partially-filled sacks 
heaped on each other, the prominences of one projecting into corresponding 
hollows in the next. The general aspect of this structure is shown in Tig. 
12, which represents a face of rock about eight feet higli and six feet broad. 
The rocks exhibiting this peculiarity are usually finely amygdaloidal, and it 
may be observed that the vesicles are grouped in lines parallel to the 
outer surface of the pillow-like block in which they occur. The diagram in 
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