You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Asphalt

Context:
Selenizza is a naturally occurring solid hydrocarbon bitumen found in the native asphalt deposit of Selenice, in Albania, the only European asphalt mine still in use. The rock asphalt is found in the form of veins, filling cracks in a more or less horizontal direction. The bitumen content varies from 83% to 92% (soluble in carbon disulphide), with a penetration value near to zero and a softening point (ring & ball) around 120 °C. The insoluble matter, consisting mainly of silica ore, ranges from 8% to 17%.

Question:
How is the rock asphalt mainly found?

veins